Michelson A. A. Light waves and their uses (1903)

As was remarked above, this sodium line is double, ?\ e., is really made up of two lines close together. The ’distance between these two lines is a convenient standard of measurement for our subsequent work. This distance is so small that a single prism scarcely shows that the line is double. As we increase the number of prisms, the lines are separated more and more widely. If, instead of a prism, we use one of the best grating spectroscopes, the two lines are separated so far that we might count sixty or eighty lines between; and this fact gives a fair idea of the resolving power of these instruments. If we have two lines so close together as to be separated by only one-hundredth of the distance between these two sodium lines, the best spectroscope will hardly be able to separate them; i. e., its limit of resolution has been reached. %

The difference in the character of the lines from different substances is illustrated in Fig. 55. The spectrum that you have just seen is a photograph from a drawing, not a photograph from a spectrum. These are from spectra. On the right is a portion of the spectrum of iroji, the other the corresponding portion of that of zinc. The enormous diversity in the appearance of the lines will be noted. Some are exceedingly fine—so fine that they are not visible at all; others are so broad that they cover ten or twenty times the distance between two sodium lines. This width of the lines depends somewhat upon the conditions under which the different substances are burned. If the incandescent vapor which sends out the lines is very fig. 54 dense, then the lines are very broad; if it is very

Interference Methods in Spectroscopy 63

rare, then the lines are exceedingly narrow. Some of the lines are double, some triple, and some are very complex in their character; and it is this complexity of character or structure to which I wish particularly to draw your attention.

This complexity of the character of the lines indicates a corresponding complexity in the molecules whose vibrations cause the light which produces these lines; hence the very considerable interest in studying the structure of the lines themselves. In very many cases — indeed, I may say, in most cases—this structure is so fine that even with the most powerful spectroscope it is impossible to see it all. If this order of complexity, or order of fineness, or closeness of the component lines is something like one-hundredth of the distance wo have adopted as our standard, it is practically just beyond the range of the best spectroscopes. It therefore becomes interesting to attempt to discover the structure by means of interference methods.

In order to understand how interference can be made use of, let us consider the nature of the interference phenomena which would be produced by an absolutely homogeneous train of waves, i. e., one which consisted of only one definite simple harmonic vibration. If such a train of waves were sent into an interferometer, it would produce a definite set of fringes, and if the mirror C (Fig. 39) of the interferometer were moved so as to